I have been an OSM mapper for some years now, but I have found that an OSM newbie has deleted some admin boundries. I have looked at the documantation to revert anothers changesets and have started to try reverting, but the revert plugin is confusing for me to download and use.

just a partial answer/comment at "revert plugin is confusing for me to download and use": This is okay. You should know what you are doing when using the revert plugin for JOSM. Otherwise it can happen that you worsen the situation. You did the right thing in asking someone else.

I've reverted the relevant changesets in 20368253 and 20368273. Are you already in contact with the new user concerned? If so, it would be useful to know (in order to improve iD documentation etc.) what they were trying to do when they did these deletions. The deletions weren't just admin boundaries - they seem to be everything in a small area.

One suggestion (for future reversion requests from other people who might see this "help" question) - you might get a quicker answer on IRC than here since help questions regularly sit for hours or longer until people see them, and if other people have edited objects aftr the problematic edits it becomes more difficult to revert.

I think it could even be a intentional deletion of someone who does not like their company to be publically mapped (compare the user name and the company name). Communication with the user is important.

I have conteacted 'Warners' and I have made them aware of their mistake and I will be helping them to delete the ways which afffect them and not the surrounding areas. I mapped this area and they were happy for me to do so, then their committee decided to delete it all. It ended up with them deleting everything be accident. I have now fixed it all according to their request and also keeping everythingelse intact.